Systems which generate images, be they IR, MR, radar or x-ray, have two different requirements. The one requirement can be seen as spatial resolution and the other dynamic resolution. The requirement for spatial resolution comes from the need to resolve small objects and the dynamic resolution from the need to be able to resolve and measure signal levels on an image surface. For example, with radar, small objects such as aeroplanes may need to be detected and, with another, weather radar, the density of a cloud may need to be established to determine whether or not it is a rain cloud. In the healthcare sector, personnel may want to look at fractures in a leg or tissue change in an organ. The one need concerns looking at a large signal difference in a small area, and the other concerns looking at a small signal difference in a large area or the absolute signal. There are many examples and, in most cases, active sensor systems can meet these needs by adapting the energy in the emitted radiation. In passive systems, however, the emitted radiation can, of course, not be controlled since that is what needs to be measured.
In the area of IR image transmission, which is called thermography or radiometry, the focus is on dynamic resolution, i.e. being able to determine the temperature of a surface. When IR is used instead of or in addition to other image generating systems, e.g. for monitoring, reconnaissance or in military applications, the focus is on spatial resolution without consideration of absolute levels. This means that one often works with different intelligent image processing solutions to increase the spatial contrast. For example, classic histogram equalisation can be applied. Other well-known methods for increasing the contrast in an image include Retinex, Unsharp Mask, AHE (Adaptive Histogram Equalisation), CLAHE (Contrast Limited Adaptive Histogram Equalisation) and other types of frequency-based high-pass filters. The disadvantage of these and other known methods in an IR context is that the information concerning the absolute signal level or temperature of an object in the scene is no longer preserved.
There are many instances of when a good image needs to have good contrast and produce radiometric information, for example, during search and rescue (SAR) operations, when one wants to be able to establish a small object at a long distance, such as someone who has fallen overboard into the sea or a walker lost in the mountains. In these examples a check also needs to be made of the temperature to avoid pursuing a buoy, a reflection, stone or suchlike. In these cases, so-called isotherms are sometimes taken. An equivalent situation exists in thermography when one wants to add spatial resolution to a radiometric image. A small change in a large area can therefore be localised and the exact position, for example, of a fracture in a pipe identified. Therefore, it is an advantage if the operator can also differentiate details and also determine the object's absolute temperature. Until now, the requirements for high spatial resolution combined with high dynamic resolution and preserved radiometric information have been inconsistent.